Sean O'Brien and Jimmy Gopperth guided Leinster to a hard-earned Heineken Cup win over Ospreys in Swansea.
The hosts have been touted as Welsh rugby's best hope of European success this season.
But O'Brien's first-half try and Gopperth's conversion and four penalties dealt them a blow.
Ospreys' Wales fly-half Dan Biggar kicked three penalties, but opposite number Gopperth's late penalty denied the hosts a losing bonus point.
The visitors were without midfield talisman Brian O'Driscoll, but the Irish province overcame his absence.
Flanker O'Brien took the man-of-the-match award for an outstanding display in defence, attack and particularly at the breakdown.
And having been preferred to Ian Madigan at 10, summer signing Gopperth's tactical efforts and goal-kicking gave Leinster a telling edge.
Ospreys back-rower Ryan Jones had an early impact on his return from a groin injury, his presence at a breakdown forcing the penalty that gave Biggar the chance to kick the game's first points.
The Wales stand-off added a second before Gopperth sent his first kick between the posts.
Steve Tandy's side had dominated the opening quarter, but wasted a good chance when hooker Richard Hibbard chose to go it alone instead of passing.
Hibbard was pulled down short of the line, and Leinster conceded a penalty.
Home skipper Alun Wyn Jones opted for a close-range scrum instead of another Biggar penalty attempt.
However, Leinster forced a penalty at the scrum from a home pack containing five British and Irish Lions.
And moments later Jamie Heaslip stole possession from centre Andrew Bishop to launch the move that ended with O'Brien going over and Gopperth converting.
New Zealander Gopperth added a second penalty to put the visitors 13-6 ahead at the break.
Biggar and Gopperth exchanged penalties amid a tense second period, but the Ospreys had little choice but to run at Leinster whenever time and space allowed.
But with O'Brien guiding a disciplined defensive effort the visitors were equal to the challenge as they kept the Ospreys at bay.
Justin Tipuric was heavily involved in one of the home side's best attacking chances.
The Wales and Lions flanker's well-timed pass sent Canadian wing Jeff Hassler away on the right.
But Gopperth's tenacious covering run and tackle halted Hassler 20 metres out.
And after former Ospreys academy player Rhys Ruddock's powerful run helped earn Leinster a penalty, the Leinster pivot gave them the 10-point winning margin.
Ospreys travel to Northampton on Sunday, 20 October in the second round while Leinster host Castres the previous day.
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Ospreys assistant coach Gruff Rees: "At key moments we made bad decisions. We were not smart under pressure, and they were the opposite. They did the right thing at the right time.
"We worked really hard to try and get some attacking momentum, but there were just some key moments.
"We had a penalty at 13-9 which we took quickly which was unforgiveable, and it meant it went to 16-9 and we were back under the pump.
"We had a couple of opportunities where we could have turned the tables on them, but we did not look after the ball, we were too shallow, too lateral in attack.
"It meant they won the race to the breakdown and they killed us there."
Ospreys: Richard Fussell; Ben John, Andrew Bishop, Ashley Beck, Eli Walker; Dan Biggar, Tito Tebaldi; Duncan Jones, Richard Hibbard, Adam Jones, Alun Wyn Jones (capt), James King, Ryan Jones, Joe Bearman, Justin Tipuric.
Replacements: Jeff Hassler for John (56), Matthew Morgan for Beck (73), Tom Habberfield for Tebaldi (58), Ryan Bevington for D Jones (50), Scott Baldwin for Hibbard (69), Aaron Jarvis for Adam Jones (62), Sam Lewis for R Jones (62).
Not Used: Ardron.
Leinster: R Kearney; F McFadden, B Macken, G D'Arcy, D Kearney; J Gopperth, I Boss; C Healy, S Cronin, M Ross; D Toner, M McCarthy; K McLaughlin, J Heaslip (capt), S O'Brien.
Replacements: A Dundon, J McGrath, M Moore, Q Roux, R Ruddock, E Reddan, I Madigan, L Fitzgerald
Replacements: Luke Fitzgerald for Macken (68), Eoin Reddan for Boss (68), J Jack McGrath for Healy (62), Martin Moore for Ross (18), Quinn Roux for McCarthy (73), Rhys Ruddock for K McLaughlin (68).
Not Used: Aaron Dundon, Ian Madigan.
Referee: Wayne Barnes (Eng)
Assistant referees: Tim Wigglesworth (Eng), Paul Dix (Eng)
TMO: Geoff Warren (Eng)
Citing commissioner: Shaun Gallagher (Eng)